Optical switch

ABSTRACT

An optical switch comprises a hub and a plurality of nodes with the hub being connected to each node by an optical communication link dedicated for clock signals and by an optical communication link dedicated or data signals. In use the hub transmits a clock signal to all of the nodes; each node re-transmits a copy of the clock signal to the hub and transmits a data signal to the hub. The hub returns each re-transmitted clock signal to its respective node and forwards a copy of each data signal to all of the nodes so that each node can receive a selected data signal by processing the re-transmitted clock signal.

[0001] This invention relates to the field of optical switches forcommunications networks and specifically optical switch fabrics.

[0002] Optical communications technology has advanced rapidly in recentyears, with transmission systems capable of terabits per second nowbeing deployed. However, advances in switching and routing technologyhave not been as dramatic, leading to system bottlenecks as signals areconverted from optical format to electronic format for processing,before being re-converted to an optical format for onwards transmission.

[0003] According to a first aspect of the present invention there isprovided an optical switch, comprising a hub and a plurality of nodes,each node being connected to the hub by first optical communication linkfor clock signals and by second optical communication link for datasignals; such that, in use: the hub transmits a clock signal to all ofthe nodes; in response to receiving said clock signal, each nodere-transmits the clock signal to the hub and transmits a data signal tothe hub; the hub transmitting each data signal to all of the nodes andreturning each re-transmitted clock signal to its respective node, eachnode processing the re-transmitted clock signal to receive a selecteddata signal. Preferably, each node generates data signals by modulatingthe received clock signal. The clock signal may comprise a plurality ofwavelength division multiplexed pulses and each data signal may comprisea plurality of wavelength division multiplexed data pulses.

[0004] Preferably, the data signal transmitted by each node has atemporal offset relative to the clock signal which is unique to therespective node. Additionally, the hub may transmit each data signal toall of the nodes and return each re-transmitted clock signal to itsrespective node, each node determining the respective temporal offsetfrom the re-transmitted clock signal to receive a selected data signal.

[0005] According to a second aspect of the present invention there isprovided a method of switching optical signals, the method comprisingthe steps of:

[0006] (a) transmitting a clock signal from a hub to a plurality ofnodes;

[0007] (b) re-transmitting the clock signal back to the hub from eachnode;

[0008] (c) transmitting a data signal from each node to the hub;

[0009] (d) returning the re-transmitted clock signal to each respectivenode

[0010] (e) transmitting all of the received data signals from the hub toall of the nodes;

[0011] (f) at one or more of the nodes, processing the re-transmittedclock signal to select a data signal.

[0012] It is preferred that in step (c) the data signal transmitted byeach node has a temporal offset relative to the clock signal which isunique to the respective node and that in steps (e) and (f), each nodedetermines the respective temporal offset from the re-transmitted clocksignal to receive a selected data signal.

[0013] The invention will now be described, by way of example only, withreference to the following figures in which;

[0014]FIG. 1 shows an outline of an optical switch according to thepresent invention;

[0015]FIG. 2 is a schematic depiction of a first embodiment of anoptical switch according to the present invention; and

[0016]FIG. 3 is a schematic depiction of a second embodiment of anoptical switch according to the present invention.

[0017]FIG. 1 shows a schematic depiction of an optical switch accordingto the present invention. The switch comprises a hub 10 which is incommunication with a plurality of nodes 20, each of which is connectedto the hub 10 by optical communication links 30.

[0018]FIG. 2 shows a schematic depiction of the connection of a hub 10to one of the nodes 20 (only one node is shown in FIG. 2 for the sake ofclarity) via optical communications links 30. Hub 10 comprises a 1×Noptical coupler 11, optical pulse source 12, N×N optical coupler 13 andoptical circulator 14. Hub 10 is connected to node 20 by opticalcommunications links 31, 32, 33 & 34. Node 20 comprises write module 200and read module 250. Write module 200 comprises phase locked loop (PLL)201, optical circulator 202, electro-absorption modulator (EAM) 203,variable optical delay 204, optical 1×2 couplers 205 & 206, fibrestretcher 207, optical receivers 208 & 209 and band pass filters 210 &211. Read module 250 comprises optical receiver 251, electro-absorptionmodulator 252, impulse generator 253, variable micro phase shifter 254,band pass filter 255, optical receiver 256 and optical 1×2 coupler 257.

[0019] Optical pulse generator 12 generates a stream of short opticalpulses, for example picosecond duration pulses, that is transmitted to1×N optical coupler 11. Optical coupler 11 has an output leg for eachnode 20 that the hub 10 is in communication with and may haveadditional, unused output legs for connection to additional nodes 20 ifthe switch is to be extended. FIG. 2 shows one particular node 20; inthis case the respective output leg of the optical coupler 11 isconnected to optical circulator 14, which forwards the pulse stream tonode 20 via optical communications link 31. For the other nodes (notshown in FIG. 2) the pulse stream is transmitted over opticalcommunication links 35 to the optical circulators (also not shown inFIG. 2) associated with those nodes. Thus, the pulse stream generated bythe optical pulse generator 11 is distributed to all of the nodes 20that are connected to the hub 10.

[0020] Hub 10 also comprises N×N optical coupler 13, the inputs of whichare connected to each of the circulators 14 in the hub 10. The outputsof the N×N optical coupler 13 are connected to the node shown in FIG. 2by optical communication link 34. Optical communication links 36 connectthe outputs of N×N optical coupler 13 to respective hubs 20 (not shownin FIG. 2). Thus, each of the hubs 20 is inter-connected so that anysignals transmitted from hub 20 to the node along optical communicationlink 31 will pass through N×N optical coupler 13 and then be sent to allof the other nodes 20.

[0021] Write module 200 of node 20 receives the optical clock pulse fromoptical communication link 31 at circulator 202. The clock pulse is sentthrough fibre stretcher 207, which controlled by the phase locked loop(PLL) 201. Two copies of the clock pulse are then made by optical 1×2coupler 206.

[0022] One of the output legs of optical 1×2 coupler 206 is connected tovariable optical delay 204 and then EAM 203. Data to be switched throughthe optical switch (i.e. to one of the other hubs connected to the node)is modulated onto each pulse of the pulse comb. Modulated data pulsesare fed into optical circulator 202 and transmitted over opticalcommunication link 31 to optical circulator 14, which directs the datapulses to the N×N optical coupler 13, which distributes the data pulsesto all of the nodes connected to the hub. Optical communication link 34carries the output of the N×N optical coupler 13 to the read module 250of the node 20 and this output will be the combination of the datapulses from the write modules 200 of each of the nodes connected to thehub. In order to prevent the various data pulses from interfering witheach other it is necessary to provide a separation mechanism. Thepreferred mechanism is time division multiplexing the different datapulses; the variable optical delay 204 adds a time delay with respect tothe received clock pulse before the data is modulated onto the clockpulses. The delay added in each different node 20 is chosen such thatdata pulses from all of the nodes can successfully coexist within thesame communication link.

[0023] The other output leg of optical 1×2 coupler 206 is connected tothe input leg of optical 1×2 coupler 205. Clock pulses are transmittedto the PLL 201 via one of the output legs of optical 1×2 coupler 205,the optical receiver 208 and band pass filter 210. The second output legof optical 1×2 coupler 205 is transmitted to read module 250 of the node20 via optical communication link 32, hub 10 and optical communicationlink 33. The clock pulse stream is returned to the write module 200 byoptical 1×2 coupler 257 and is fed to the PLL 201 via optical receiver209 and band pass filter 211.

[0024] In order for the data pulses from the different nodes to retainthe desired time separation it may be necessary to change the pathlength that the data pulses propagate over; this allows for thecompensation of changes in temperature, especially when nodes are notco-located and thus may be subject to different environmentalconditions. The PLL controls the fibre shifter to decrease the opticalpath length to enable the data pulse from the respective node to ‘speedup’ and to increase the optical path length to enable the data pulsefrom the respective node to ‘slow down’.

[0025] Read module 250 receives clock pulses from optical communicationlink 33 and data pulse combs(comprising a data pulse from each of thenodes) from optical communication link 34. Clock pulse are returned towrite module 250 via one of the outputs of optical 1×2 coupler 257 (seediscussion above). The second output of optical 1×2 coupler 257 isconnected to optical receiver 256. The electrical signal generated byoptical receiver 256 is passed through band pass filter 255 and variablemicrowave phase shifter 254. As the data pulses are time divisionmultiplexed to provide a relative time gap between each data pulse, itis possible to determine the arrival time of any of the data pulses at agiven node from the arrival of the clock pulse from the write module ofthe node. This relative time delay is used to determine the shiftapplied by microwave phase shifter 254 such that the impulse generator253 can drive the EAM 252 to receive the required data pulse from thedata comb, by gating the EAM.

[0026] It will be readily understood that the present invention relatesto a fabric for an optical switch or router. The method by which asuitable path through the switch (or router) is selected (i.e. thepairing if an input port and an output port), o the method of by whichport contention is avoided is immaterial and does not effect the workingof the present invention. Although the above description hasspecifically described a number of components it should be noted that itis the function of the described device that is critical, rather thanit's structure. For example, EAM 252 could be replaced by any otheroptical device that could be used to provide a gate function to ‘drop’the selected data pulse from the data comb.

[0027] Optical communication links 31, 32, 33 & 34 that connect eachnode to the hub should have the same optical characteristics so as tominimise the differences in optical path length and other propagationphenomena. The inventor has realised that this result can preferably beachieved by the use of ‘blown fibre’ optical cables, in which 4 opticalfibres are tightly bound in a jacket (see EP-A-O 186 753 and EP-B-O 521710).

[0028] The capacity of the switch fabric will be limited by the temporalwidth of the clock and data pulses used in the network and the width ofthe guardbands which will be necessary to prevent adjacent pulses frominterfering with each other. The device limitations which will limit theswitch capacity will be the capability of the optical pulse generator 12and the capability of the gating devices to ‘drop’ a desired pulsewhilst maintaining a sufficient extinction ratio such that the gatingdevices do not add noise by inadvertently ‘dropping’ a fraction of thepulses that are adjacent the desired pulse.

[0029] A second embodiment of the present invention is shown in FIG. 3to provide a multiple wavelength optical switch. Optical pulse generator12 generates a clock data comb comprising a number of pulses each havinga different optical wavelength rather than a single clock pulse asdescribed above. In a similar manner as described above in relation tothe first embodiment, the clock data comb is sent from the hub to eachof the nodes; each of the nodes re-transmits the clock data comb back tothe hub along with a modulated data comb (which has had the correctamount of temporal delay added); the hub transmits all of the data combsto each of the nodes such that a desired data pulse can be dropped fromone of the data combs.

[0030] In order to achieve this performance, it is necessary to modifythe structure of the write module and the read module of the nodes (seeFIG. 3). Before the data can be modulated over the different wavelengthpulses which comprise the clock comb it is necessary to divide the clockcomb into its constituent pulses. Arrayed waveguide 212 is connected tothe output of variable optical delay 204 so that each of the differentclock pulses can be separated (although FIG. 3 shows only 4 outputs fromthe arrayed waveguide (AWG) it will be understood that this is anarbitrary value and that the number of wavelengths used will vary withthe switch capacity that is desired). Each of the AWG outputs isconnected to an electro-absorption modulator so that the desired datacan be modulated over the clock pulse to generate a data pulse (for thesake of clarity only one of these EAMs is shown in FIG. 3). The outputsof these EAMs are connected to AWG 213 which re-combines the differentdata pulses to form a multiple wavelength data comb which is transmittedto the hub.

[0031] In order for the read module 250 to drop a data pulse from themultiple wavelength data comb it is necessary to add 1×N splitter 258and arrayed waveguide 259 to the read module (see FIG. 3); additionallythe one set of the devices required to drop a data pulse (opticalreceiver 251, electro-absorption modulator 252, impulse generator 253,variable micro phase shifter 254, band pass filter 255 and opticalreceiver 256) must be provided for each of the different wavelengthsbeing used in the switch. One of the outputs of coupler 257 is connectedto 1×N splitter 258 (where N is the number of wavelengths being used inthe switch), which creates a copy of the clock pulse for each of thesets of devices needed to drop the data pulses. For the sake of clarityFIG. 3 shows both 1×N splitter 258 and AWG 259 as having only fouroutputs and only one set of receiving devices is shown. AWG 259 splitsthe data comb into its constituent data pulses, each of which has anassociated set of receiving devices such that the read node cansimultaneously drop all N pulses from a data comb.

[0032] The use of the multiple wavelengths turns the switch into awavelength-and time-division multiplexed switch, further increasing thecapacity of the switch. However, the use of the different wavelengthscauses an additional problem as the different wavelengths will propagateat different speeds in the optical fibres 30. In order to preventwavelength-dependent temporal skew (i.e. some of the data pulses gettingout of step with other pulses from the same data comb) it is preferredto arrange the waveguides of AWGs 212 and 259 such that the wavelengthsthat incur the greatest delay in the fibres have the shortest opticalpath through the AWG and the wavelengths that incur the smallest delaywill have a greater path length through the AWG. Suitable selection ofthe AWG path lengths enables any temporal skew to be removed at theoutput of the AWG such that all of the data pulses are temporallyaligned before being either modulated by EAM 203 or dropped by EAM 252.

1. An optical switch, comprising a hub and a plurality of nodes, eachnode being connected to the hub by first optical communication link forclock signals and by second optical communication link for data signals;such that, in use: the hub transmits a clock signal to all of the nodes;in response to receiving said clock signal, each node re-transmits theclock signal to the hub and transmits a data signal to the hub; the hubtransmitting each data signal to all of the nodes and returning eachre-transmitted clock signal to its respective node, each node processingthe re-transmitted clock signal to receive a selected data signal.
 2. Anoptical switch according to claim 1, wherein each node generates datasignals by modulating the received clock signal.
 3. An optical switchaccording to claim 1 or claim 2, wherein the clock signal comprises aplurality of wavelength division multiplexed pulses and each data signalcomprises a plurality of wavelength division multiplexed data pulses. 4.An optical switch according to any preceding claim, wherein the datasignal transmitted by each node has a temporal offset relative to theclock signal which is unique to the respective node.
 5. An opticalswitch according to claim 4, wherein the hub transmits each data signalto all of the nodes and returns each re-transmitted clock signal to itsrespective node, each node determining the respective temporal offsetfrom the re-transmitted clock signal to receive a selected data signal.6. A method of switching optical signals, the method comprising thesteps of: (a) transmitting a clock signal from a hub to a plurality ofnodes; (b) re-transmitting the clock signal back to the hub from eachnode; (c) transmitting a data signal from each node to the hub; (d)returning the re-transmitted clock signal to each respective node (e)transmitting all of the received data signals from the hub to all of thenodes; (f) at one or more of the nodes, processing the re-transmittedclock signal to select a data signal.
 7. A method of switching opticalsignals according to claim 6, wherein in step (c) the data signaltransmitted by each node has a temporal offset relative to the clocksignal which is unique to the respective node.
 8. A method of switchingoptical signals according to claim 6, wherein in steps (e) and (f), eachnode determines the respective temporal offset from the re-transmittedclock signal to receive a selected data signal.